FIG. 6(a) is a plan view illustrating a conventional microwave integrated circuit (hereinafter referred to as MIC) and FIG. 6(b) is a cross-sectional view taken along line 6B--6B of FIG. 6(a). In the figures, reference numeral 1 designates a dielectric substrate. A conductor 2 is disposed on the rear surface of the dielectric substrate 1. A microstrip line conductor (or a coplanar line conductor) 3 using the conductor 2 as a grounding conductor is disposed on the surface of the dielectric substrate 1. Through-hole electrodes 16 and 16' penetrate portions of the dielectric substrate 1 and connect to the conductor 2. A semiconductor chip 7 is disposed on the through-hole electrode 16 and connected to the microstrip line conductor 3 by wires 15. The semiconductor chip includes transistors, diodes, resistances, and the like, which are fabricated on a semiconductor substrate. The through-hole electrode 16' is connected to the microstrip line conductor 3 by wires 15.
FIG. 7(a) is a plan view illustrating a conventional microwave monolithic integrated circuit (hereinafter referred to as MMIC) and FIG. 7(b) is a cross-sectional view taken along line 7B--7B of FIG. 7(a). In the figures, reference numeral 8 designates a semiconductor substrate. A transistor 12 is disposed in a surface region of the semiconductor substrate 8. A conductor 2 is disposed on the rear surface of the semiconductor substrate 8. A microstrip line conductor 3 using the conductor 2 as a grounding conductor is disposed on the front surface of the semiconductor substrate 8 and connected to the transistor 12. The source of the transistor 12 and the end portions of the microstrip line 3 are grounded by via-hole conductors 5.
The conventional MIC of FIGS. 6(a) and 6(b) has the following drawbacks.
(1) Although FIGS. 6(a) and 6(b) illustrate a single stage amplifier including one transistor 7, in case of a two or more stage amplifier including a plurality of semiconductor elements, the number of nodes connecting the semiconductor elements to the microwave transmission line increases and a lot of wires are needed, increasing production costs and reducing reliability.
(2) The wire 15 is used for connecting the microwave transmission line 3 to the semiconductor element 7, and the inductance of the wire is not negligible in a high frequency band, i.e., at sub-millimeter wavelengths. In addition, variations in the lengths of the wires 15 cause variations in the characteristics of the MIC, i.e., amplification characteristics, VSWR (Voltage Standing Wave Ratio) of the input, gain, output power, noise factor, and the like.
(3) The semiconductor element 7 is disposed on the through-hole conductor 16 for grounding and heat radiation. However, heat generated in the semiconductor element 7 does not diffuse transverse to the through-hole conductor 16, and thermal stress is caused by the difference in the thermal expansion coefficients between the material of the through-hole conductor 16 and the material of the dielectric substrate 1, resulting in poor heat radiation of the semiconductor element that reduces saturation power output (maximum power output) and power application efficiency.
On the other hand, the MMIC of FIGS. 7(a) and 7(b) has the following drawbacks.
(1) Since the microstrip line conductor 3 is disposed on the semiconductor substrate 8 which has a large dielectric loss, the line loss increases. For example, a GaAs substrate has a dielectric loss tangent (tan.delta.) of 0.001.
(2) Since the semiconductor substrate 8 must be as thick as 100 microns so that the microwave transmission line 3 has a desired characteristic impedance, the heat radiation of the semiconductor element is poor, reducing the saturation output and power application efficiency.
(3) Since the process steps for fabricating the semiconductor element 12 and the microstrip line conductor 3 on the semiconductor substrate 8 are serially carried out, the production yield, which is determined by multiplying the yields of the respective steps, is reduced. In addition, the uneven surface of the substrate due to the presence of semiconductor elements causes uneven deposition of photoresist in subsequent steps for producing passive circuit elements. Therefore, the application of the photoresist must be divided into two steps to reduce the unevenness of the substrate surface which requires an advanced surface flattening technique and severe production conditions, resulting in an increase in the production cost.